1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying apparatus or a printer, utilizing an electrophotographic process or an electrostatic recording process for obtaining an image by developing an electrostatic image formed on an image bearing member with a developer, and an image control method in such image forming apparatus.
2. Related Background Art
It is already known, for example in an image forming apparatus of electrophotographic process, to develop a latent image formed on an image bearing member with a developer in a developing device, thereby rendering it visible as a toner image. Also there is known a developing device utilizing a two-component developer containing toner and carrier.
In a developing device utilizing a two-component developer, it is important to maintain a constant toner concentration, namely a constant mixing ratio T/(T+C) of the toner (T) and the carrier (C) (hereinafter also referred to as T/(T+C) ratio). For this reason, an image forming apparatus employing the two-component developing method is equipped with an auto toner replenisher (ATR). As a toner concentration control method in such an auto toner replenisher, namely a method of measuring and controlling the T/(T+C) ratio of the developer, there is employed a method of forming a patch image which is an image pattern of a reference density for controlling the toner concentration (hereinafter also called “toner concentration control patch”), on the image bearing member, and detecting the T/(T+C) ratio from the measurement of the image density of the patch (such toner concentration control method being hereinafter referred to as “patch detection method”).
For example in an image forming apparatus for forming full-color images at a high speed, a patch image formation and a density detection for each image in the continuous image formation will lower the image output speed and will promote stains in the apparatus. However, in a digital image forming apparatus, it is possible, in a continuous image forming operation, to estimate the toner consumption by adding image information signals and to replenish the toner in succession, based on the estimated consumption amount (such toner concentration control method being hereinafter called “video count method”). Such video count method is used in combination with the patch detection method (for example cf. Japanese Patent Application Laid-open No. H06-011965).
Also in an image forming apparatus of an electrophotographic method, a density of an output image may become higher or lower than an expected density because of various factors including, for example, environmental conditions such as temperature and humidity at the printing operation, a temperature change or a deterioration in a photosensitive drum or a fixing device of the printer, and a residual toner amount. Such phenomenon is particularly conspicuous in an image of an intermediate density level. In order to compensate such fluctuation in the output density characteristics caused by a variation in the image output conditions and to obtain an appropriate density in the output image, there is executed a control on image parameters (image density correction control), which is different from the toner concentration in the developer. As such image density correction control, there is known, for example, a following density correction characteristic control (γLUT control), which corrects density correction characteristics (γLUT) for correcting an image information signal that is used for forming the electrostatic image. More specifically, there is formed a patch image which is an image pattern corresponding to an image signal of a predetermined density level (such patch image being hereinafter also called “density correction characteristic controlling patch”), and a density of such patch image is measured. Then the measured density of the patch image is compared with a standard density at a corresponding density level. Then a density correction table (γLUT) for correcting the density level of the image data is so prepared that the output density characteristics have a predetermined property (for example linearity) (for example cf. Japanese Patent Application Laid-open No. H10-16304).
However the toner concentration control and the density correction characteristic control are independent controls as described above and have been executed independently for stabilizing the image density.
Therefore, in an image forming apparatus equipped with both of the toner concentration control and the density correction characteristic control, both controls may compete with each other to result in an excessive control thereby leading to an instability in the image density. For example, in case the toner concentration control and the density correction characteristic control form patch images (toner concentration controlling patch and density correction characteristic controlling patch) almost at the same time and such patch images are judged to have a low density (low toner deposition amount), the toner concentration control executes a toner replenishment while the density correction characteristic control makes a correction on the density correction characteristics (γLUT) to elevate the density, whereby the image density may become excessively high as a result.
It is ideal to at first execute the toner concentration control to obtain an appropriate T/(T+C) ratio and then to execute the density correction characteristic control, but such sequence requires a complex control and a long control time, thereby significantly deteriorating the productivity of the image forming apparatus.
Consequently, there are required an image forming apparatus and an image control method, capable of correlating the toner concentration control and an image parameter control different from the toner concentration control (such as density correction characteristic control) thereby maintaining the image density in a simpler and shorter control.